This invention is concerned with improvements in magnetic recording media and particularly with the magnetic recording media capable of making magnetic recordings of high density and high resolution in the present video systems.
In order to improve the recording density of a magnetic recording medium and to increase the sensitivity thereof, such a non-magnetic support as polyethylene terephthalate, cellulose triacetate base or the like has been coated with a magnetic layer comprising ferromagnetic powder and binders both of which have been kneaded together. The ferromagnetic powder should have high residual magnetic flux density Br, ratio of the Br to saturation magnetic flux density Bm and coercive force Hc, respectively. The binders include vinyl chloridevinyl acetate copolymer, butyl acrylate-acrylonitrile copolymer, cellulose resin, polyurethane resin and the like. To serve as the ferromagnetic powder, it is advantageous from the viewpoint of magnetic characteristics such as Br/Bm, Br, Hc and the like to use a cobalt adsorbed iron oxide such as Co adsorbed .gamma.-Fe.sub.2 O.sub.3 or metal magnetic powder. However, the conventional magnetic powder is an acicular crystal and if they are more finely granulated to increase the resolution power, there can be obtained those having the longer axis of 0.3 to 0.9.mu. and the shorter axis of 0.05 to 0.1.mu. in length, and the ratio of such longer axis to the shorter axis, i.e., the acicular ratio is of the order of 5 to 10. Meanwhile, the Hc of these conventional metal magnetic powder is increased as they are granulated to saturate the head of video deck to a value too great to use with the present video systems.
In magnetic powder of conventional Co adsorbed iron oxide having a high acicular ratio, such as Co-adsorbed .gamma.-Fe.sub.2 O.sub.3, the saturation magnetization .sigma.s is decreased as the powder is finely granulated, and the high acicular ratio magnetic grains are easily broken to cause an uneven distribution of Hc in the magnetic layer, that is a cause of image transfer between laminated magnetic layers.
When using powder having a high acicular ratio, it is required to strengthen the oriented magnetic field because of the necessity of increasing the degree of orientation. As a result, the grains are stuck together, so that the surface of the magnetic layer is roughened. Such powder also tend to decrease in saturation magnetization .sigma.s and not to be suitable for a high sensitivity and high density recording, as they become finer. It is a matter of course in the case of Co-adsorbed iron oxide that the decrease of saturation magnetization .sigma.s can be retarded if the amount of Co is increased. It is however undesired in practical use because other problems are derived therefrom. Namely, if the Co concentration is too high, thermal instability and pressure instability are caused with a resultant change of the magnetic characteristics. Further, Co is expensive in cost, that is not suitable in practical application.
With respect to recorders of a coaxial circles type or a spiral type recording system using ferromagnetic powder whose ratio of average longer axis to average shorter axis in length is within the scope of 1.2 to 3.0, there has been disclosed in Japanese Patent Examined Publication No. 32408/1982. However, this type of magnetic recording media, e.g., the so-called floppy-disks, have so far been used, and the Hc of such well-known ferromagnetic powder is merely 20 to 400 Oe.
In the systems adopted in the present coaxial circles type or spiral type magnetic recording media, the Hc has been prescribed in a limited range, and in the said publication there was also no disclosure to improve the Hc of ferromagnetic powders in particular as well as to adopt to systems having a wider range of Hc. In the meanwhile, ferromagnetic powder having even higher Hc has been demanded in the new systems of this type of field, however, .gamma.-Fe.sub.2 O.sub.3 disclosed in the said Publication cannot satisfy such demand. In addition, the conventionally known Co-containing .gamma.-Fe.sub.2 O.sub.3 has had a problem in the thermal stabilization and has hardly been put into practical application.
In addition, Japanese Patent Publication Open to Public Inspection (hereinafter called Japanese Patent O.P.I. Publication) No. 98135/1982 discloses a tape type magnetic recording medium bearing at least not less than two magnetic layers thereon. It discloses that the acicular ratio of the ferromagnetic powder used in this magnetic recording medium is higher than 1 and lower than 3. In such a magnetic tape, a video tape in particular, having not less than two magnetic layers as disclosed in the described Publication, there are scattering of voices, brightness, picture noises and the like.
Further, dusts and dirts are apt to adhere to the coating surface of a magnetic layer to cause the so-called "Drop-out" phenomenon. This patent publication does not describe any proper Hc but merely describe the recording media in which .gamma.-Fe.sub.2 O.sub.3 having an extremely low coercive forces of 280 to 330 Oe or Fe-powder having a normal Hc of 900 to 1050 Oe.
In addition, Japanese Patent O.P.I. Publication No. 501053/1082 discloses a magnetic recording element made of a metal oxide having a metallic dope such as, preferably, Co, having an acicular ratio of not higher than 15 and preferably of 2 to 8 and the Hc of not lower than 450 Oe and preferably of not lower than 650 Oe. However, there is a problem of thermal stability in such magnetic recording media using a metal oxide of a metallic dope such as Co dope .gamma.-Fe.sub.2 O.sub.3.